Conference reception attendees, house guests, picnic goers, and party attendees at stand up events often need two hands to juggle a drink and a plate while walking around and perhaps selecting hors d'oeurves and appetizers from servers. This can be disconcerting when a table is not available to hold one of them while using the other. Juggling a plate in one hand and a drink in the other often makes it awkward to greet others at the event.
Also, when two people are socializing at a reception, one using a cup and a plate and the other just using a cup, when one of them needs to go to the restroom or go somewhere without the cup, then the other can hold the two cups on one plate and still be able to eat while the other person is gone without their cup.
Trays and other gadgets that are separate components are sometimes thoughtfully provided by a host so that guests can hold the plate and the cup in one hand. But usually, these separate components are an afterthought and when available are something that needs to be cleaned and put away or are an additional piece of garbage that needs to be disposed of properly.